Suction rolls are used particularly in paper machines, to remove water from the paper web. The suction roll has a perforated shell and a stationary suction box inside, so that the part of the jacket next to it is subjected to suction. The seal, i.e. the seal strip is arranged in a holder in the structure of the suction box, this generally being a U-shaped holder, in the bottom of which, and possibly on the sides of which there are loading hoses, for pressing the seal initially against the shell. Later, the suction holds the seal against the shell.
The suction box, which is elongated in the longitudinal direction of the roll, is bounded by long side seals, the first of which, i.e. the seal strip on the wet side, is narrower than the second, i.e. the seal strip on the dry side. In addition, there are ends seals at the ends of the suction box, set at a small gap from the shell. U.S. Pat. No. 5,746,891 discloses the seal construction of one such suction roll.
Seal wear has become a quite important problem. In addition, the noise and loss of power caused by the seals are important factors. Though wear resistance is thus a primary requirement in seals, a low noise output and power demand would also be desirable properties.
Seal strips, seals in brief, are of vulcanized rubber-graphite compounds. The typical configuration of a seal strip is a continuous piece, which is the same length as the suction box (3-12 m). Multi-piece seal strips exist, but these require special machining and installation. The material wears during operation and must be replaced at relatively frequent intervals. The nature of the material is that it is a rigid, brittle structure. The seals are sent to the mill in rigid containers with space for the entire length of the strip. The length and rigidity of the package result in considerable shipping and storage costs. The brittleness of the seal material results in a lowered yield in manufacturing and breakages during shipping and installation.
During operation, the seal strips are loaded against the inner shell of the roll by means of so-called loading hoses. Though the flexibility of the loading means permits some alignment of the seal strip from end to end, the rigidity of the seal strip prevents local conformity with the roll shell. Therefore all irregularities due to the manufacture of the shell, or deflection can be compensated for only with the aid of wear. As the seal is intended to be a critical element, its useful life is shortened.
The most common material for constructing a seal strip is a vulcanized rubber-graphite compound. It is used in the vast majority of rolls today.
Publication U.S. Pat. No. 5,649,719 (Beloit) discloses the use of graphite impregnated with resin for the manufacture of a suction roll seal strip. Publication U.S. Pat. No. 4,714,523; Sawyer, discloses a seal-strip construction, in which a PTFE strip is set inside a graphite strip, in order to improve its sliding properties. Publication U.S. Pat. No. 4,915,787 (Cline Company) discloses a seal-strip construction, in which a long seal strip is formed from short pieces, the ends of which are specially shaped to form overlapping joints between the strips. Publication U.S. Pat. No. 5,876,566 (Appleton) discloses a seal strip, which is formed from nitrile rubber, graphite, carbon black, PTFE, and optionally phenol resin. U.S. Pat. No. 2,893,487 (Beloit) discloses a suction-box solution, in which, in the rearward seal strip, grooving is formed against the shell on the trailing edge of the strip. This reduces the detrimental effects, such as noise, of a sudden change in pressure.
A combination of graphite and rubber is desirable because it provides good lubricating properties, does not wear excessively in normal operation, and is easy to manufacture. It can be manufactured in all the lengths required by papermaking equipment.
All rubber graphite seals strips are made in the following manner:                A rubber graphite mixture is milled to an acceptable consistency for extrusion        The compound is extruded to the approximate shape of the end product, or of a part product        The shaped strips are vulcanized to the final rigid condition        The rigid strips are machined to their final shape.        
Fundamentally, all rubber graphite strips have the same physical characteristics, are made the same way, and perform similarly. They are all limited by shipping and handling constraints, due to their rigid and brittle nature.
The usual vulcanizing temperature is about 150° C. and the processing time is 3 hours. Additives are used, for instance, to create a suitable chemical structure in the end product. Certain additives, for example, sulphur, affect the number of carbon links. Some additives act as accelerating agents.
Other materials have been tried as seal strips, but none have been as successful are rubber graphite.